Antenna input circuits



July 18, 1950 J. s. COHEN ANTENNA INPUT CIRCUITS 2 Sheets-Shea*l l Filed June 25, 1947 July 18, 1950 .1. s. COHEN ANTENNA INPUT CIRCUITS 2 Sheets-Sheet 2 Filed June 25, 1947 @www Patented July 18, 1950 UNITED STATES PATENT OFFICE ANTENNA INPUT CIRCUITS Jeffrey S. Cohen, Los Angeles, Calif., assignor to Aveo Manufacturing Corporation, Cincinnati, Ohio, a corporation of Delaware Application June 25, 1947, Serial No. 756,992

(Cl. Z50-20) plication, specifically to radio receivers of the type suitable for either frequency modulation (FM) or short wave (SW) signal reception.

In spite ofthe great progress made by the radio industry during the last half century and the many outstanding contributions to science made by research Workers in this field, many problems have persistently sought solution. One problem which has troubled the industry for many years and stimulated the best efforts of radio research Workers is the prevention of undesired signal radiation consequent upon the coupling of a local oscillator and an antenna circuit to a frequency conversion stage.

`The most popular type of receiver is the superheterodyne and it poses a serious problem. It comprises an antenna-ground system for intercepting radio-frequency Wave-energy, a variably tunable local oscillator for developing local signals, a frequency converter for utilizing the intercepted Wave signals and the local signals to.l develop intermediate-frequency carrier signals, an

intermediate-frequency amplifier for amplifying the last-named signals, a detector for deriving the intelligence'from those signals, an audiofrequency amplifier for amplifying the detected f I, signals and a transducer for converting the detected signals into sound. This type of receiver is very selective, efficient and therefore in popular demand, primarily because the intermediatefrequency amplifier can be tailor made always to effectively and faithfully amplify signals of the same intermediate frequency.

It is customary to amplify the received radiofrequency signals before they are applied to the frequency converter. One preselector or radiofrequency amplifier stage is usually employedfor this purpose. However, a frequency converter can also be used as an amplifier and it is highly .desirable to utilize that characteristic by coupling the antenna to the frequency converter. This step poses one problem solved by the present invention. The local oscillator is inherently a transmitter and it is always coupled to the frequency converter. Coupling the antenna to the converter in accordance with the prior art involved coupling the local oscillator to the antenna also. Then the receiver would function as a transmitter and there 'Would be radiated out from the local oscillator undesired signals which impaired reception at other stations. With the advent of extremely high frequency reception, when the dimensions of the antenna became so small as to be comparable to the short Wave lengths of such extremely high frequency signals, this problem became very serious. The smaller the antenna dimensions and the higher the frequency of the local oscillations, the more efficient becomes the antenna as a radiator of these undesired signals. So the prior art failed fully vto utilize the above-mentioned amplification characteristic of the converter and interposed a radio-frequency amplifier between the antenna and the frequency converter, in order to uncouple or isolate the local oscillator from the antenna. Even this expedient did not successfully solve the problem because it did not prevent ground currents from by-passing the radio-frequency Stage and proceeding from the local oscillator to the antenna.

Other disadvantages of the preselector stage Were that it required tuning, increased production expense and costs to the consumer, and constituted a source of service troubles.

The disadvantages of the prior art practice are excessive costs land the introduction of switch contacts in the radio-frequency circuits. Such contacts seriously impair the performance of frequency modulation circuits and should be elimi nated wherever possible.

Another problem raised by the prior art is that of discriminating against received signals of intermediate frequency before they reach the mixer or conve-rter. It is customary to provide separate Wave traps or the like for performing this function. It is desirable to utilize the existing short Wave antenna input circuit elements to perform this task While the receiver is receiving frequency modulated Waves. Such a step would save the cost of additional circuit elements and achieve maximum utilization of necessary existing circuit elements. In accordance with the invention shown in my copending divisional patent application, Serial No. 37,579, entitled Antenna Input Circuits and assigned to the same assignee as the present invention and application, there is provided an arrangement for accomplishing this objective.

It is accordingly a basic object of the invention to provide means here shown for purposes of illustration as a transformer including a primary coupled to the antenna and a balanced bridge-secondary having one arm coupled to the converter tube and both arms coupled to the primary in opposition to each other and a center tap connected to the oscillator for utilizingthe oscillator in such a way as to voltage-amplifyv the received signals, to apply both receivedsignals and local oscillations to the converter, and

at the same time to nullify the eliects` ofk the* local oscillations on the antenna.

A further object of the invention contemplates the providing of a simple, inexpensive balanced trar'lsformer structure for the purpose set forth.

For a better understandingfof the present invention, together with other and further objects and capabilities thereof, reference is made to the 'accompanying drawings in which there isillustrated a signal 'translating system embodying a preferred illustrative' form of the invention.

the drawings:

Fig, 1 is ya circuit diagram of a FM SW receiving system including a balanced FM input transformer and bridge network, a dual purpose antenna circuit and a combined FM IF rejector and SW input transformer circuit in accordance with the invention; Fig. 2 is a perspective View of a preferred form of balanced FM input transformer, serving-also as a mounting for other circuit components hereinbelow described; Figs. 3,

'4; 5' and 6 are, respectively right side, front, back land bottom views of the Fig. 2 transformer; and Fig. '7 is: an elevational sectional View taken on line 'I--l of Fig. 4, looking in the direction of the arrows.

Referring now specifically to Fig. 1, there is illustrated a signal translating system including a balanced frequency-modulation signal input transformer and bridge network so arranged as Alio-prevent radiation of undesired oscillator signals', a dual purpose antenna circuit, and a combined rejector and double tuned band pass antcnna short wave signal input lcircuitall in accordance with the invention. These features are shown as incorporated in a receiver adapted for frequency-modulation (FM) s-ignal reception or short-wave (SW) broadcast signal reception.

For the interception of electromagnetic-Wave energy there is provided an antenna I0, adapted to function as a dipole during reception ofv frequency-modulation (FM) signals. 'I'his antenna is'con'nected by a pair of conductors to the terminals of the primary I2 of a balanced input transformer I4. This primary is a rea-ctance or inductance in the form of a simple coupling loop. The primary andrsecondary are electrostatically shielded from each other by a grounded shield I5. According to one feature of the present invention and for a purpose hereinafter explained, the secondary I6 comprises-two loop or inductance portions Il and I8, conductively joined at a center tap 21.

Transformer I4 is over coupled so as to pass the entire 20 m. c. Wide FM band.

The primary I2 is tuned to resonance at approximately the center frequency of the FM band, ninety-eight (98) megacycles, by a capacitor I9 shunted across the primary, and the sec-,

'developed' by the local oscillator.

4 ondary I6 is tuned to approximately the same frequency by a capacitor 20, shunted across the secondary.

The output from transformer I6 constitutes the received FM signal input applied to the control electrode circuit of a mixer or converter tube 2|.

AA conventional local oscillator 22a may be employed to generate high frequency signals which are also applied to an input circuit of the converter and therein beat against the received FM signal input to develop intermediate-frequency signals, in accordance with a, heterodyne process well knownin the art.

O ne important problem solved by the present invention is the prevention of the application, tothe antenna, of signals from the local oscillator, while permitting both the received signal input' and the local oscillator signals to be applied to the mixer yStage without the intervention of' a radio-frequency amplifier. It Well knownt to those' skilled in the art that a local oscillator inherently functions as a transmitter. When the lantenna input circuit and the oscillator output circuit are coupled' to a common stage by prior art expediente', they are effectively coupled together, soV that'the antenna functions as a radiator of the electromagnetic wave energy Undesired signals are therefore radi-ated. 'I'hese signals constitute a nuisance of a high order, interfering with-regularly broadcast signals and impairing theenjoyment of reception by other niembers of the radio audience.

In successfully solving this problem, the circuit of secondary I6 is arranged as a balanced bridge, o-ne branch' of which comprises an rinductive reactance or winding I1, (capacitor 2li and thevinpu't impedance of vacuum tube 2l, and the other branch of which comprises an inductive reactance or Winding I8, and a resistor 22 and'a capacitor 23,v comprising `a dummy load. Accordingly, capacitor 23 is placed in circuit between groundand the junction of capacitor 2l! land'vvinding I8. The cathode-control grid interelectrode" capacitance of tube 2l and grid Ycapacitor 24' are serially arranged in circuit between. ground and the junction of capacitor 2D and'winding I.'I. The dummy load resistor 22 is shunted across winding I8. Windings or reactancesV I 1=` andv I8 are ratio arms of the bridge. The third arm is effectively the input impedance of tube 2l. The fourth arm is effectively the dummy load. Gridresistor 25 is connected between a suitable source of automatic-volume control` bias (not shown) and the junction of capacitor 24 andthe grid. Primary I2 and secondary I6 are magnetically coupled but capacity coupling therebetween is prevented in this illustrative embodiment by the shield I5'.

The .signals from `local oscillator 22a, are applied'throughA coupling Vcapacitor 26 to the center tap 2'! between'windings I1 and I 8 or some equivalent potential point, thus being impressed ybetween center tap and ground'. The local oscillator signals appearing in winding I'I are equal inmagnitude and opposite in phase to those appearing in Winding I8 and since oscillator signals in both windings I'I and IB are appliedini opposition to primary I2, no currents are induced in primary I2 by the local oscillator signals. Magnetic fluxl interlinkages tending to induce undesired voltages by secondaryv action on primary I2 are cancelled out. In other Words; the coupling tothe primary I2 isbalanced `180 degree phase split and both portions "out, so far as the'oscillator lisfconcerned, and

This is accomplished by adjusting capacitor 23 for a voltage null across the antenna terminals, the oscillator voltages applied to ratio arms I'I and I8 then being equal and opposite in phase. Viewed 'in another way, the oscillator signals are are applied in opposition to primary I2.

y A double-tuned band pass antenna input circuit for SW signals, novel in respects hereinafter pointed out, is provided. Specifically, a tuned primary circuit comprising winding 28 and capacitor 29 in one branch, and capacitor 30 in a parallel branch, is interposed between a center tap on primary I2 and ground. It will be noted that the output of the mixer stage including tube 2| is 'coupled to an intermediate frequency amplier 3|. `The output signals of the mixer stage are of the intermediate frequency, say 10.7 megacycles, for FM operation.

It is well known to those skilled in the art that #stron'gsignals of intermediate frequency from undesired stations may be picked up by the trans'- mission line Illa connecting the dipole antenna I to the input transformer I4. Interference of this type is reduced on the FM band by arranging the vprimary of the antenna input circuit so as to quencies. Thus, in the Fig. 1 system, inductance .218@ andcapacitor 29 are in series resonance at the "intermediate frequency for FM operation and attenuate intermediate frequency signals picked .up on the transmission lines Illa.

L .The FM operation of the circuits just described is! next considered. The balanced half-wave ,dipole II) intercepts electromagnetic wave energy and delivers it to the balanced input transformer circuit I4.

The balanced, shielded, double-tuned band pass, FM input transformer I4 translate with gain the received signal voltage and rejects undesired signals outside the FM band. At the same time it prevents undesired radiation of the local oscillator signals. As noted above, were the lastmentioned signals not attenuated, as seen by the antenna, they would cause interference with other receivers in the vicinity. The mixer stage including tube 2| amplies and converts the received FM signals to signals of intermediate frequency, say 10.7 megacycles.

The invention makes commercially practicable the entire omission of a radio-frequency amplier stage and dispenses with the necessity of Variably tuning the input circuit, with large resultant savings in costs to the consumer. Service troubles are also considerably reduced.

The IF signal output of tube 2| is applied to the intermediate frequency amplifier 3| in any conventional manner. It will be understood that the units (not shown) in cascade with and following unit 3| may constitute the usual remaining units of a superheterodyne FM receiver: one or more IF stages, a discriminator, an audio amplifier, a power amplifier, and a loud speaker or transducer.

Before considering the short wave (SW) operation of the Fig. 1 system, certain other circuit elements will be described. The SW doubletuned, high gain, band pass antenna input circuit transformer 35 comprises also a secondary 32,v tuned by a capacitor 33 in shunt therewith. The primary 28 and condenser 29 act as a Seriesresonant rejector circuit for the FM IF frequency, as stated above (being tuned to 10.7 megacycles, approximately the middle of the band-spread SW band). The primary 28 and secondary 32 are inductively overcoupled and so arranged that the two spaced secondary response peaks occur at 31 and 25 meters, say. The band covers the 31 meter and 25 meter international SW broadcast stations.

Resistor 34, between winding I2 and ground, simply functions as a static leak to bleed off antenna charges to ground.

Coming new to the SW operation of the abovedescribed system, it will be observed that the antenna terminal of the double tuned input transformer circuit including elements 28, 29 and 3U is connected to primary I2. The dipole I0 functions during SW operation as a T-type antenna, with both conductor portions in parallel, the impedance of primary I2 being low at that frequency range. The intercepted wave energy is applied to the primary of SW input transformer 35, and translated to the control electrode of tube 2|.

For SW operation a single-pole double-throw. switch 35 is so set as to couple the secondary cir` cuit of transformer 35 to the input circuit of tube 2|. For FM operation, the switch is so set that the secondary circuit of transformer I4 is coupled to the inputcircuit of tube 2| Oscillator 22 may have two ungrounded terminals, one for FM operation andthe other for SW operation. During SW operation transformer I4 and the high frequency oscillator terminal of unit 22a are in opencircuit relation to tube 2| and `the switch 36 couples the lower-frequency oscillator terminals to the input of tube 2 I. 3

In Figs. 2 to '7, inclusive, there ,is illustrated a form of transformer unit I4 which has been found eminently satisfactory from a practical commercial standpoint. The circuit components shown therein have the same reference numerals as those shown in Fig. 1. The unit comprises a pair of grounded non-magnetic metallic plates 40 and 4|, held in fixed spaced relation by hollow non-magnetic metallic posts 42 and 43, secured to plate 4| as by soldering or brazing. To avoid closed eddy current paths and magnetic shielding effects the free ends of the posts are insulated from plate 40 as by grommets 44 and 45. The mounting assembly is completed by an insulating plate 46, to which spaced inturned lugs 41-50, integral with plates 40 and 4I, are secured as by riveting. Conductive straps 5| and 52 connect lugs 41, 48, and 49, 50 respectively. Integral with plate 4I are mounting lugs 59 and 60.

The primary I2 is a U-shaped wire having legs projecting through insulating tubes 53 and 54, the tubes being concentrically disposed within posts 43 and 42, respectively. The central portion of the primary straddles a part of plate 4|, the primary being insulated by washers 55-58. The primary leads are soldered to and support ceramic condenser I9.

The secondary comprises two loops I'I and I8, symmetrically mounted on each side of the posts, the common axis of the loops being perpendicular to the plane of the posts 42 and 43. The outer loop leads are brought out through insulating plate 46 and are soldered to and support ceramic condenser 20, The inner loop leads are brought out through posts in plate 46 to a conductive cen- It has been determined that, from a commercial and production standpoint, the fab'ove described transformer is Vsimple, A:ineimensive leiicient and durable. Other transformers can, rof course, be usedin'thelig. 1 system. The one descriloedlias been found y.particularly effective,

foilowing villustrative circuit parameters havebe'en foundsuitable fn a Vsuccessful embodiinentfoft-he invention:

-fItis `intended in the appended claims to obtain the @advantages A'of the 'fdoctrineof equivalents. l=I cla-im:

In an electromagnetic wave receiverzof the type comprising an input Vtransformer Shaving va .primary and a vcenter-tapped secondary, a mixer tube having a single :anode and cathode and Ycontrol electrode elements, said secondarybeing Icoupled to said cathodea'nd control electrode, andan oscillator eiectively coupled between said cathodev and the tap on said secondary, whereby -two paths lare in parallel with said oscillator, one comprising one half of said secondary andthe-ef fe'ctive input impedance of said tube and rincluding capacitance and resistance, `and the 'other path comprising the remainder lof said secondary, athe improvement which resides in means for lproducingfan oscillator voltage null across said secondary comprising acapactorin series With-said secondary and a resistor in parallel with said remainderfof said secondary, said .capacitor andresistor being in-such other pathlandhavingsuch valuesfas to equalize'said paths.

JEFFREY S. COHEN.

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